1. Field of the Invention
The present invention relates to the field of radiation oncology for malignant tumors and the like. Specifically, the present invention provides methods and systems for planning delivery of radiation therapy by means of a single-arc dose of radiation.
2. Description of the Related Art
Conformal radiation therapy is an important procedure available to the physician for the treatment of malignant tumors. Such therapy is used for eradicating or shrinking tumors that are relatively inaccessible to other modes of treatment such as surgical excision. However, because the ionizing radiation that is administered is damaging to both healthy and malignant tissue, it is important to confine the effect of the irradiation to the target tissue, to the extent possible, while sparing the adjacent tissue by minimizing irradiation thereto. To achieve this goal, various techniques of irradiating a target tumor with a defined beam of ionizing radiation have been devised.
Although simple masking techniques using radiation absorbing materials have some utility, techniques using radiation beams directed at the target tissue from various angles about a patient have come to be preferred. Such beams are often provided from a radiation source, e.g., x-ray photons or high-energy electrons, mounted on a rotating gantry, so that the radiation source revolves in a generally circular path while providing a beam of radiation directed generally at the isocenter of such a path. A patient is positioned within the circular path, preferably with the tumor located, to the extent possible, at the isocenter for receiving the maximum dose of radiation as the source is revolved. The cross-sectional shape and size of the radiation beam is typically varied as the source is positioned at different angles by rotation of the gantry in order to assure, to the extent possible, that the radiation is incident on the tumor itself and not on adjacent healthy tissue.
A number of techniques have been developed with the intention of providing for maximum absorption of radiation within the tumor while minimizing exposure of adjacent healthy tissue. Intensity-modulated radiation therapy (IMRT) was developed based on the principle that for a given tumor, there is a set of preferred ways to direct the radiation to it. More radiation can be sent through some beam angles than others and within the same beam angle, there are preferred locations through which the radiation is directed to the tumor. Computer-assisted treatment planning systems have been developed to take advantage of such preferred angles and locations, e.g., by varying the intensity of the radiation beam across the radiation fields, in order to accomplish better treatment regimens.
Consequently, intensity-modulated radiation therapy (IMRT) has been widely adopted as a new tool in radiation therapy to deliver high doses of radiation to the tumor while providing maximal sparing of surrounding critical structures. Both rotational and gantry-fixed IMRT techniques have been implemented clinically using dynamic multileaf collimation (DMLC) (1-6).
In gantry-fixed IMRT, multiple radiation beams at different orientations, each with spatially modulated beam intensities, are used (1-2, 4). The beams may be administered to the patient in a single transverse plane as the source revolves around the patient (coplanar) or may be shifted axially with respect to the patient (non-coplanar). Rotational IMRT, typically, administered by a continuously revolving source that is also moved axially along the patient, as it is currently practiced, mainly employs temporally modulated fan beams (3). Although the quality of IMRT treatment plans has steadily improved, the plans tend to be relatively complicated, which makes for a somewhat inefficient delivery of treatment. Consequently, labor-intensiveness has been one of the drawbacks of IMRT. Furthermore, in general, a large number of different complex field shapes is often needed, which also compromises the efficiency of the treatment and can result in an increased number of collimator artifacts (7). Nevertheless, while long-term clinical results of IMRT treatments are still limited, initial results appear very promising, and with increased use of IMRT, more encouraging results are emerging.
U.S. Pat. No. 5,818,902 to Yu teaches the use of overlapping multiple arcs to deliver modulated beam intensities around the patient which is called intensity-modulated arc therapy (IMAT). Delivery of the radiation during overlapping multiple arcs achieves modulated beam intensities at all angles around the patient (5-6). However, IMAT has not been widely adopted for clinical use. In IMAT, the intensity distributions are first optimized using a treatment planning system for tightly-spaced beam angles every 5-10 degrees all around the tumor. These intensity distributions are then approximated by a stack of uniform intensity segments with different cross sectional shapes. These stacks of uniform beam segments at all beam angles are then sequentially administered as the radiation source describes multiple rotational arcs while the beam cross sections are defined at each angular position by a multi-leaf collimator (MLC) with its leaves moved by a computer-programmed controller to provide a sequence of predetermined apertures.
The inverse planning procedures used to determine beam cross-sections and intensities when planning IMRT and IMAT treatments have typically required the user to predetermine the number of beams to use and their orientations. This limitation can significantly affect the quality of the treatment plans because the most preferred angle might be completely missed. Rotational IMRT does not have such problems since all angles are considered in the plan.
Furthermore, because the delivery of IMAT requires the use of multiple (4-11) arcs, each of which may take 1 to 2 minutes to deliver, the total treatment time is similar to that of fixed beam IMRT treatments. It is also relevant that, even when intensity distribution is determined for densely-spaced beam angles, i.e., 5 to 10 degrees, and sequenced for delivery in a limited number of multiple arcs, the resultant distribution of radiation absorption in the tumor can still only approximate that which would be provided by optimized beam intensities and cross-sections, because each of the planned beams may require significant variations that cannot be accommodated by the plan as executed by the equipment. As a result, the final IMAT plan is almost always degraded from the unconstrained optimized plan.
One attempt to solve this problem is a method for optimizing IMAT using Direct Aperture Optimization, in which the shape and weight of the apertures contained in one or more arcs are optimized simultaneously (8). A similar method (9), who showed that a single arc optimization using a method similar to Direct Aperture Optimization could generate satisfactory treatment plans for a simple case. In both methods, a limited number of beam angles were used to illustrate the principle. For complicated cases, single-arc optimization over a limited number of angles cannot generate plans that rival fixed-beam IMRT plans (8). Using such methods, it is prohibitive with today's technology to optimize the rotational delivery with more beam angles, because pencil beams must be calculated for all the beam angles (8-9).
Another concern with current radiation treatment planning methods is that no method developed hitherto can create a rotational IMRT plan that consistently rivals fixed beam IMRT without requiring beam intensity modulation. For simple cases it was demonstrated that a single optimized arc can yield results as good as those of fixed-field IMRT, while, for more complicated cases, such as head and neck cases with multiple targets, such an approach would not work well, and intensity modulation is required (8). Although allowing the planned dose rate to vary with gantry angle provides a new degree of freedom, such relaxation of a restraint is not sufficient in itself to establish that a single arc using the described optimization method can replace multiple-arc IMAT. Treatment planning using current optimization schemes requires intensity modulation consistently to rival fixed beam IMRT (10). Another method that can achieve very good efficiency utilizes optimization based on the direct aperture optimization method of Shepard et al (10). However, although the optimization starts with a limited number of fields and the connectedness of the field shapes can be ignored, as the optimization progresses, constraint to force the shape-connectedness will compromise the quality of he treatment plans. As the result, one would know what the ultimate plan quality is like.
Thus, there is a recognized need in the art for improved radiation therapy planning methods that enable greater efficiency in delivery of radiation therapy. More specifically, the prior art is deficient in methods and systems for single-arc dose radiation therapy. The present invention fulfills this long-standing need and desire in the art.